1. Field of the Invention
This invention relates to a drive train of a vehicle, in particular of a mobile machine, with a drive motor, in particular an internal combustion engine, and a traction drive driven by the drive motor. The traction drive has a primary unit driven by the drive motor and a hydraulic work system is driven by the drive motor. The hydraulic work system has at least one hydraulic pump driven by the drive motor. When the vehicle is operated in deceleration or braking, the hydraulic pump of the hydraulic work system delivers into a hydraulic pressure accumulator to recover energy.
2. Description of the Current Technology
Self-propelled machines, in particular industrial trucks, agricultural machines, forestry machines and construction machinery, for example, excavators, wheeled loaders and telescopic loaders, tractors, combine threshers, forage harvesters, and sugar beet or potato diggers, have a drive train with a drive motor that is generally in the form of an internal combustion engine which drives a traction drive and a hydraulic work system for the work functions of the mobile machine. There is at least one hydraulic pump driven by the drive motor which supplies the hydraulic work system.
A drive train of this type can be provided with a hydraulic pressure accumulator and a hydraulic machine unit that is in communication with the pressure accumulator to deliver torque into the drive train and to absorb torque from the drive train. As a result of the charging of the pressure accumulator during the deceleration or braking of the traction drive and/or the lowering of a load, the kinetic energy of the vehicle and/or the potential energy of a raised load can be recovered. By operating the hydraulic machine unit as a motor with the hydraulic fluid from the pressure accumulator, in addition to the internal combustion engine, a torque can be delivered into the drive train in the form of a booster drive. The hydraulic machine unit driven by the pressure accumulator thereby forms a hydrostatic booster drive of the drive train and makes possible in particular a booster drive in which, when there is a demand for power, the drive motor is assisted by the additional torque which is delivered by the hydraulic machine unit into the drive train. The energy recovered in the pressure accumulator during the deceleration or braking of the vehicle can again be made available by the hydraulic machine unit to the drive train when the machine is re-accelerated, for example, so that the energy for the acceleration of the machine need no longer be supplied exclusively by the drive motor.
The hydrostatic booster drive formed by the pressure accumulator and the hydraulic machine unit therefore make it possible to reduce the fuel consumption of the internal combustion engine. In addition, the power of the internal combustion engine can be reduced as a result of the presence of the hydrostatic booster drive. A downsizing of the internal combustion engine results in advantages, in particular in compliance with the exhaust standards imposed by law on the internal combustion engine.
The hydrostatic booster drive formed by the pressure accumulator and the hydraulic machine unit can also be used as a hydraulic starter of a drive motor which is in the form of an internal combustion engine to reduce the fuel consumption, in connection with a start-stop function for the internal combustion engine, by shutting down the idling internal combustion engine during pauses or interruptions in the work and by automatically starting it up again in the event of a demand for torque by a work function or by the traction drive.
DE 10 2008 028 547 A1 describes a drive train of a mobile machine in which energy is recovered during deceleration and braking by charging a hydraulic pressure accumulator. The hydraulic machine unit, which is in communication with the hydraulic pressure accumulator, can be used as a booster drive and as a hydraulic starter of the internal combustion engine. The hydraulic machine unit in DE 10 2008 028 547 A1 connected with the pressure accumulator is in the form of a radial piston machine that is operated in an open circuit and takes up relatively little space in the axial direction, is located in the drive train between the drive motor and the primary unit of the traction drive, and is simultaneously used for the supply of the hydraulic work system. When the machine is decelerated or braked, the radial piston machine that is used to supply the hydraulic work system functions as a pump and charges the pressure accumulator. When the radial piston machine is operated as a motor, it is driven by the hydraulic fluid from the pressure accumulator to provide a boost to the drive motor or so that the radial piston machine can be used as the starter of the drive motor which is in the form of an internal combustion engine. In DE 10 2008 028 547 A1, the radial piston machine is connected on a first side to the pressure accumulator and on a second side to the tank. However, a radial piston machine of this type that can be operated as a pump and motor in an open circuit to supply the hydraulic work system requires a great deal of construction effort and expense to manufacture to make it possible to operate the machine as a pump and a motor, since it must be possible to adjust the radial piston machine across the null point in both directions to achieve the reversal of delivery direction necessary for operation as a pump or as a motor. The special design of the hydraulic pump of the hydraulic work system as a radial piston machine that can be adjusted in both directions and its simultaneous utilization as a hydraulic machine of the hydrostatic booster drive therefore means that a high level of effort and expense is required in the design and construction of the drive system.
DE 32 47 335 describes a drive train with a hydrostatic booster drive which is formed by a hydraulic pressure accumulator and a hydraulic work pump operated in an open circuit. When the vehicle is decelerated, the pressure accumulator can be charged with hydraulic fluid by the hydraulic work pump and energy can be recovered. To feed a booster torque into the drive train, the pressure accumulator is connected with the suction side of the hydraulic work pump, as a result of which the hydraulic work pump operates as a motor. A hydraulic work pump of this type operating in an open circuit such as, for example, an axial piston pump with a fixed displacement or variable volumetric displacement which is in communication by the suction connection with the tank, is generally designed for a high suction limit speed. The suction connection with the suction channel thereby has a correspondingly large cross section, whereby during operation as a motor, in which the pressure accumulator is connected with the suction connection of the hydraulic work pump, the housing generally does not have sufficient strength to withstand the pressures required to achieve the desired additional torque in the drive train. In addition, when there is a connection between the suction connection of the hydraulic work pump with the pressure accumulator when the hydraulic work pump is being operated as a motor, it is necessary to make the suction connection and the suction channel resistant to high pressure, which further increases the time, effort, and expense involved in the design and construction of the pump.
In DE 10 2008 028 547 A1 and DE 324 47 335 A1, when the machine is decelerating or braking, during which process the hydraulic pump of the hydraulic work system delivers into the pressure accumulator, the brake power and thus the kinetic energy to be extracted in the pressure accumulator is determined from the volumetric displacement flow of the hydraulic pump and the pressure in the hydraulic accumulator. In a machine with a hydrostatic traction drive in a closed circuit, during deceleration and braking there is a change in pressure in the hydrostatic traction drive, as a result of which the drive motor, which is generally an internal combustion engine, is driven by the traction drive and its speed of rotation is increased when it is operating in the tractor mode. The volumetric displacement flow delivered by the hydraulic pump of the hydraulic work system during deceleration and braking thereby corresponds to the set delivery quantity and the speed of the drive motor operating in tractor mode. In machines of this type, the hydraulic pump of the hydraulic work system is limited to the medium pressure range and the maximum volumetric displacement of the hydraulic pump is adapted to and designed for the requirements of the hydraulic work system.
In particular on mobile machines with a high operational weight, such as on the mobile machines described in DE 10 2008 028 547 A1 and DE 32 47 335 A1, the maximum power installed on the hydraulic pump of the hydraulic work system may no longer be sufficient to absorb the kinetic energy during a deceleration or braking of the machine and to achieve the desired deceleration.